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Metallicities and the kinematics of low-mass stars are key parameters
when studying the structure and evolution of our Galaxy. The most
accurate way to determine metallicity is through spectroscopy, while
the most precise way to get distances is astrometry. Parallaxes from
the Hipparcos mission are available for many thousands of stars, but
their accuracy rapidly deteriorates beyond 0.1 - 0.2 kpc away from the
Sun. For large-scale stellar surveys, such as SDSS/SEGUE, APOGEE, and
RAVE, which aim to observe the Galaxy from its innermost bulge regions
to the outer halo at distances larger than 50 kpc (Fig. 1), scientists
have to find another solution.
The only alternative is to use the information in a stellar spectrum,
combining the luminosity data with photometry and stellar evolution
models. A prerequisite for this approach are physically realistic models
of radiative transfer in stellar atmospheres. Then researchers obtain
accurate physical parameters of a star when applying them to observed
spectra.
In the past decades, spectroscopy of low-mass stars relied on
simplified models that were based on the assumptions of both local
thermodynamic (LTE) and 1D hydrostatic equilibrium. Because the models
are very widely used for the analysis of large datasets, including
SDSS and RAVE, the principal question is whether such an 1D LTE
approach is accurate.
Recently, scientists at MPA worked together with researchers in Spain
and Sweden to devise a new method for computing stellar parameters and
distances. This method includes new physical effects (such as non-LTE
radiative transfer) in stellar models, which so far have not been
included in a single calculation to date.
The scientists found that with the new method, the parameters obtained
from the stellar spectra change: the metal-poor stars become warmer,
more metal-rich and less evolved, i.e., their surface gravities
increase, accompanied by a decrease in luminosity. As a consequence,
the stars become fainter and this in turn leads to much shorter
distances. For most of the stars, the distance thus decreases by
10-50%. That has a major impact on the volume distribution of stars (a
comparison of previous with the new accurate results is shown in
Fig. 2).
These improvements in the physics of radiative transfer models have a
large impact on the distribution functions of stellar samples. In a
magnitude-limited survey (such as RAVE), where more metal-rich
unevolved stars dominate the nearby sample and metal-poor luminous
giants are predominantly observed at larger distances, classical LTE
analysis will systematically over-estimate distances, placing stars
progressively further than they are. This would cause the unphysical
smearing of the metallicity distribution function (Fig. 3, black
area), as well as the stretching of the distance scale. Clearly, the
effects will be more prominent in lower-metallicity stars.
Having verified the new method, the team is now ready to analyse much
larger stellar samples, such as e.g., SDSS/SEGUE. These will provide
radically new information about the properties of stellar populations
in the Milky Way. In particular, they will shed new light on the
controversy about the origin of our Galactic halo that is currently
being debated.
Maria Bergemann (MPA), Aldo Serenelli (ICE/CSIC, Barcelona), Greg
Ruchti (MPA/Lund Observatory, Sweden)
References
Aldo Serenelli, Maria Bergemann, Gregory Ruchti, Luca Casagrande,
2013, accepted for publication in MNRAS, arXiv:1212.4497
Maria Bergemann, Aldo Serenelli, Gregory Ruchti, 2013, proceedings
IAUS289, Cambridge University Press
Gregory Ruchti, Maria Bergemann, Aldo Serenelli, Luca Casagrande,
Karin Lind, MNRAS, 2012, doi:10.1093/mnras/sts319, arXiv:1210.7998
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